In this project, autoradiography at the light and electron microscope levels will be used to localize sites of androgen binding (cytoplasmic receptor, CR; and androgen binding protein, ABP) in the seminiferous tubules of the rat testis. It is of interest to know whether these components occur throughout the extensive cytoplasm of the Sertoli cell, whether they are restricted to this cell type, and whether their distribution and amount varies over the cycle of the seminiferous epithelium. Labeled testosterone will be administered to the testis, and after a wash period, the tissue will be prepared for LM and EM autoradiography. Since conventional techniques of preparation would cause extraction or movement of the steroid, the method involves freezing the tissue (without fixation), freeze-drying, vapor fixation with osmium tetroxide in a vacuum, and vacuum-embedding in epoxy resin. Once the localization of CR and ABP has been worked out in the rat, the method will be applied to some other mammalian species (some of which apparently do not make ABP).